Previous attempts to copolymerize aziridines (ethylenimines) and vicinal epoxides or episulfides, including both the substituted and nonsubstituted monomers, have been generally unsuccessful in that the reaction product was not an alternating copolymer but was rather a mixture of homopolymers, or a polyalkylenimine having pendant hydroxyalkyl groups, as illustrated by: (1) Y. Oshiro et al., Chem. Abstracts, 64: 14291b; 64: 14292c; and 66: 19069p; (2) C. G. Overberger et al., Journal of Polymer Science: Part A. Vol. 2, 2481 (1964); O. C. Dermer, Wright Air Development Center(WADC) Technical Report 55-447(1956), Publication Board (PB) 121,796. Generally a catalyst, such as BF.sub.3, was considered essential in the reaction.